a. Field of the Invention
This invention relates to a system and method for automated correction of preset characteristics of a throttle position sensor in an electronic controller for an automatic transmission having shifting solenoids operated in accordance with a vehicle speed signal and a throttle position signal.
b. Description of the Related Art
In general, an electronic controller for an electronically-Controlled automatic transmission controls shifting, acceleration, fuel injection, and the like in accordance with predetermined patterns based upon vehicle speed signals and throttle position signals.
To this end, a throttle position sensor provides position signals of an engine throttle (the depression of an accelerator pedal) which are needed to control shifting, acceleration, fuel injection, and the like. As is illustrated in FIGS. 4 and 6, the throttle position sensor is in the form of a potentiometer in which arcuate strips 2 of resistor film are deposited on a ceramic substrate 1 and a metal contact (throttle-position-indicating contact) 3 slides on the resistors 2 to produce a voltage which varies in accordance with the position of the sliding contacts on the resistors. A spindle 4 on which the sliding metal contact 3 is mounted is connected via a lever to a throttle valve shaft mounted on a side of an engine throttle body, so that the spindle 4 rotates in correspondence with movement of the throttle valve. To permit accurate detection of a fully closed position of the throttle valve, an additional sliding contact (idling contact) 5 is Suitably mounted to detect an idling position of the throttle valve.
The signal which indicates the throttle position is output as an analog voltage as depicted in FIG. 5. This analog throttle position signal is converted into a digital signal, for example as shown in FIG. 6, by an A/D converter 6 as an input interface. The A/D converter 6 includes an operational amplifier 7, a D/A converter 8, a sequential comparison register 9 and a serial output buffer 10. The digital signal so obtained is then input into an electronic controller (not shown).
Generally, throttle position sensors, as normally fabricated, vary in their throttle position output voltage characteristic. In order to reduce deviations in shifting points and lockup points of automatic transmissions, the throttle position sensor is first adjusted or set to produce, for example, 10 V at the 100% throttle open position. The throttle is next moved to its 0% open position, and it is determined if the throttle position sensor output falls between tolerance limits of the output voltage that should be produced at the 0% throttle open position. If the output falls outside the tolerance limits, the throttle position sensor setting or mounting is readjusted.
However, when the conventional throttle position sensor is adjusted at either the 100% or 0% throttle open position, a variation on the opposite side (namely 0% or 100% throttle open position) cannot be avoided as shown in FIG. 7. For example if line a in FIG. 7 represents the designed output voltage characteristic of the throttle position sensor, an output voltage (V) of V.sub..theta. is supposed to be produced when the throttle opening is .theta..sub.2 %. If the throttle position sensor actually has a voltage output response characteristic as represented by line b, the controller then determines that the throttle opening corresponding to the output voltage V.sub..theta. is .theta..sub.2 % instead of the actual throttle opening of .theta..sub.1 %.
Since shifting of gears and engagement or release of a lockup clutch in an automatic transmission are conducted when the throttle position reaches preset shifting points and lockup points set relative to the 0% throttle open position, a shifting point or lockup point preset at the .theta..sub.2 % throttle open position is undesirably changed to the throttle open position.
Further problems also arise, inter alia, when the length of the throttle cable varies due to a change in temperature or the throttle position output voltage characteristic varies due to a change in resistance caused by the passage of time.
To cope with the above problems, the throttle position sensor is adjusted so that, as shown in FIG. 8, the output voltage from the throttle position sensor is V.sub.100 at the 100% throttle open position. Upon starting a program, V.sub.0 is stored in a memory device as an initial voltage data point for the 0% throttle open position. At the initial setting, a throttle position of .theta..alpha.% is calculated by a microcomputer from a detected voltage V.alpha. in accordance with the following formula: ##EQU1##
However, the throttle position output voltage characteristic may be different, for example, due to a mounting error of the throttle position sensor, a change in the length of the throttle cable due to a change in temperature or the like or a change in resistance due to aging. When a voltage V.sub.0 ' which is greater than the previously stored voltage data point V.sub.0 for 0% throttle open position is detected, it is conventional practice to store V.sub.0 ' as a new voltage data point for the 0% throttle open position in the memory device and to then calculate a throttle position .theta..alpha.' in accordance with the above formula (1) wherein V.sub.0 ' is substituted for V.sub.0.
As described above, it is conventional practice that when the throttle position output voltage characteristic has changed to update a preset reference value to the resulting changed value itself. If an incorrect value such as produced by noise is input, this incorrect value is stored, causing the computation of incorrect throttle positions.